Download How do we know more CO2 is causing warming?

This is the print version of the Skeptical Science article 'Increasing CO2 has little to no effect', which can be found at http://sks.to/greenhouse.
How do we know more CO2 is causing
warming?
What The Science Says:
An enhanced greenhouse effect from CO2 has been confirmed by multiple lines of
empirical evidence.
Climate Myth: Increasing CO2 has little to no effect
"While major green house gas H2O substantially warms the Earth, minor green house
gases such as CO2 have little effect.... The 6-fold increase in hydrocarbon use since 1940
has had no noticeable effect on atmospheric temperature ... " (Environmental Effects of
Increased Atmospheric Carbon Dioxide)
Predicting the Future
Good scientific theories are said to have ‘predictive power’. In other words, armed only with a
theory, we should be able to make predictions about a subject. If the theory’s any good, the
predictions will come true.
Here’s an example: when the Table of Elements was proposed, many elements were yet to be
discovered. Using the theory behind the Periodic Table, the Russian chemist Dmitri Mendeleev
was able to predict the properties of germanium, gallium and scandium, despite the fact they
hadn’t been discovered.
The effect of adding man-made CO2 is predicted in the theory of greenhouse gases. This
theory was first proposed by Swedish scientist Svante Arrhenius in 1896, based on earlier
work by Fourier and Tyndall. Many scientist have refined the theory in the last century. Nearly
all have reached the same conclusion: if we increase the amount of greenhouse gases in the
atmosphere, the Earth will warm up.
What they don’t agree on is by how much. This issue is called ‘climate sensitivity’, the amount
the temperatures will increase if CO2 is doubled from pre-industrial levels. Climate models have
predicted the least temperature rise would be on average 1.65°C (2.97°F) , but upper
estimates vary a lot, averaging 5.2°C (9.36°F). Current best estimates are for a rise of around
3°C (5.4°F), with a likely maximum of 4.5°C (8.1°F).
What Goes Down…
The greenhouse effect works like this: Energy arrives from the sun in the form of visible light
and ultraviolet radiation. The Earth then emits some of this energy as infrared radiation.
Greenhouse gases in the atmosphere 'capture' some of this heat, then re-emit it in all
directions - including back to the Earth's surface.
Through this process, CO2 and other greenhouse gases keep the Earth’s surface 33°Celsius
(59.4°F) warmer than it would be without them. We have added 42% more CO2, and
temperatures have gone up. There should be some evidence that links CO2 to the
temperature rise.
So far, the average global temperature has gone up by about 0.8 degrees C (1.4°F):
"According to an ongoing temperature analysis conducted by scientists at NASA’s
Goddard Institute for Space Studies (GISS)…the average global temperature on
Earth has increased by about 0.8°Celsius (1.4°Fahrenheit) since 1880. Two-thirds
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of the warming has occurred since 1975, at a rate of roughly 0.15-0.20°C per
decade."
The temperatures are going up, just like the theory predicted. But where’s the connection
with CO2, or other greenhouse gases like methane, ozone or nitrous oxide?
The connection can be found in the spectrum of greenhouse radiation. Using high-resolution
FTIR spectroscopy, we can measure the exact wavelengths of long-wave (infrared) radiation
reaching the ground.
Figure 1: Spectrum of the greenhouse radiation measured at the surface. Greenhouse effect
from water vapour is filtered out, showing the contributions of other greenhouse gases (Evans
2006).
Sure enough, we can see that CO2 is adding considerable warming, along with ozone (O3) and
methane (CH4). This is called surface radiative forcing, and the measurements are part of the
empirical evidence that CO2 is causing the warming.
...Must Go Up
How long has CO2 been contributing to increased warming? According to NASA, “Two-thirds of
the warming has occurred since 1975”. Is there a reliable way to identify CO2’s influence on
temperatures over that period?
There is: we can measure the wavelengths of long-wave radiation leaving the Earth (upward
radiation). Satellites have recorded the Earth's outbound radiation. We can examine the
spectrum of upward long-wave radiation in 1970 and 1997 to see if there are changes.
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Figure 2: Change in spectrum from 1970 to 1996 due to trace gases. 'Brightness temperature'
indicates equivalent blackbody temperature (Harries 2001).
This time, we see that during the period when temperatures increased the most, emissions of
upward radiation have decreased through radiative trapping at exactly the same
wavenumbers as they increased for downward radiation. The same greenhouse gases are
identified: CO2, methane, ozone etc.
The Empirical Evidence
As temperatures started to rise, scientists became more and more interested in the cause.
Many theories were proposed. All save one have fallen by the wayside, discarded for lack of
evidence. One theory alone has stood the test of time, strengthened by experiments.
We know CO2 absorbs and re-emits longwave radiation (Tyndall). The theory of greenhouse
gases predicts that if we increase the proportion of greenhouse gases, more warming will
occur (Arrhenius).
Scientists have measured the influence of CO2 on both incoming solar energy and outgoing
long-wave radiation. Less longwave radiation is escaping to space at the specific wavelengths
of greenhouse gases. Increased longwave radiation is measured at the surface of the Earth
at the same wavelengths.
These data provide empirical evidence for the predicted effect of CO2.
Basic rebuttal written by GPWayne
Update July 2015:
Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial
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